On Wednesday, 2023-02-15 at 11:54:09 +02, Alvaro Karsz wrote:
> This patch makes several improvements to the notification coalescing
> feature, including:
>
> - Consolidating virtio_net_ctrl_coal_tx and virtio_net_ctrl_coal_rx
> into a single struct, virtio_net_ctrl_coal, as they are identical.
> - Emphasizing that the coalescing commands are best-effort.
> - Defining the behavior of coalescing with regards to delivering
> notifications when a change occur.
> - Stating that the commands should apply to all the receive/transmit
> virtqueues.
> - Stating that every receive/transmit virtqueue should count it's own
> packets.
> - A new intro explaining the entire coalescing operation.
>
> Signed-off-by: Alvaro Karsz <[email protected]>
> ---
> v2:
> - Add the last 2 points to the patch.
> - Rephrase the "commands are best-effort" note.
> - Replace "notification" with "used buffer notification" to be
> more consistent.
> v3:
> - Add an intro explaining the entire coalescing operation.
>
> device-types/net/description.tex | 63 ++++++++++++++++++++------------
> 1 file changed, 39 insertions(+), 24 deletions(-)
>
> diff --git a/device-types/net/description.tex
> b/device-types/net/description.tex
> index 1741c79..2f4835e 100644
> --- a/device-types/net/description.tex
> +++ b/device-types/net/description.tex
> @@ -1514,15 +1514,14 @@ \subsubsection{Control Virtqueue}\label{sec:Device
> Types / Network Device / Devi
> If the VIRTIO_NET_F_NOTF_COAL feature is negotiated, the driver can
> send control commands for dynamically changing the coalescing parameters.
>
> -\begin{lstlisting}
> -struct virtio_net_ctrl_coal_rx {
> - le32 rx_max_packets;
> - le32 rx_usecs;
> -};
> +\begin{note}
> +In general, these commands are best-effort: spurious notifications could
> still arrive.
> +\end{note}
This is quite vague. How about:
The behaviour of the device in response to these commands is
best-effort: the device may generate notifications more frequently than
specified, for example after less than \field{max_packets} packets have
been processed or less than \field{usecs} usecs time has elapsed.
>
> -struct virtio_net_ctrl_coal_tx {
> - le32 tx_max_packets;
> - le32 tx_usecs;
> +\begin{lstlisting}
> +struct virtio_net_ctrl_coal {
> + le32 max_packets;
> + le32 usecs;
> };
>
> #define VIRTIO_NET_CTRL_NOTF_COAL 6
> @@ -1532,49 +1531,65 @@ \subsubsection{Control Virtqueue}\label{sec:Device
> Types / Network Device / Devi
>
> Coalescing parameters:
> \begin{itemize}
> -\item \field{rx_usecs}: Maximum number of usecs to delay a RX notification.
> -\item \field{tx_usecs}: Maximum number of usecs to delay a TX notification.
> -\item \field{rx_max_packets}: Maximum number of packets to receive before a
> RX notification.
> -\item \field{tx_max_packets}: Maximum number of packets to send before a TX
> notification.
> +\item \field{usecs} for RX: Maximum number of usecs to delay a RX
> notification.
> +\item \field{usecs} for TX: Maximum number of usecs to delay a TX
> notification.
> +\item \field{max_packets} for RX: Maximum number of packets to receive
> before a RX notification.
> +\item \field{max_packets} for TX: Maximum number of packets to send before a
> TX notification.
> \end{itemize}
>
> -
> The class VIRTIO_NET_CTRL_NOTF_COAL has 2 commands:
> \begin{enumerate}
> -\item VIRTIO_NET_CTRL_NOTF_COAL_TX_SET: set the \field{tx_usecs} and
> \field{tx_max_packets} parameters.
> -\item VIRTIO_NET_CTRL_NOTF_COAL_RX_SET: set the \field{rx_usecs} and
> \field{rx_max_packets} parameters.
> +\item VIRTIO_NET_CTRL_NOTF_COAL_TX_SET: set the \field{usecs} and
> \field{max_packets} parameters for all the transmit virtqueues.
> +\item VIRTIO_NET_CTRL_NOTF_COAL_RX_SET: set the \field{usecs} and
> \field{max_packets} parameters for all the receive virtqueues.
> \end{enumerate}
>
> -\subparagraph{RX Notifications}\label{sec:Device Types / Network Device /
> Device Operation / Control Virtqueue / Notifications Coalescing / RX
> Notifications}
> +\subparagraph{Operation}\label{sec:Device Types / Network Device / Device
> Operation / Control Virtqueue / Notifications Coalescing / Operation}
> +
> +The device sends a used buffer notification once the notification conditions
> are met, if the notifications are not suppressed as explained in
> \ref{sec:Basic Facilities of a Virtio Device / Virtqueues / Used Buffer
> Notification Suppression}.
> +
> +When the device has non-zero \field{usecs} and non-zero \field{max_packets},
> it starts counting usecs and packets upon receiving/sending a packet.
> +The device counts packets and usecs for each receive virtqueue and transmit
> virtqueue separately.
> +In this case, the notification conditions are met when \field{usecs} usecs
> elapses, or upon sending/receiving \field{max_packets} packets, whichever
> happens first.
> +
> +When the device has \field{usecs} = 0 or \field{max_packets} = 0, the
> notification conditions are met after every packet received/sent.
> +
> +\subparagraph{RX Example}\label{sec:Device Types / Network Device / Device
> Operation / Control Virtqueue / Notifications Coalescing / RX Example}
>
> If, for example:
> \begin{itemize}
> -\item \field{rx_usecs} = 10.
> -\item \field{rx_max_packets} = 15.
> +\item \field{usecs} = 10.
> +\item \field{max_packets} = 15.
> \end{itemize}
>
> -The device will operate as follows:
> +A device with a single receive virtqueue will operate as follows:
>
> \begin{itemize}
> \item The device will count received packets until it accumulates 15, or
> until 10 usecs elapsed since the first one was received.
> \item If the notifications are not suppressed by the driver, the device will
> send an used buffer notification, otherwise, the device will not send an used
> buffer notification as long as the notifications are suppressed.
> \end{itemize}
>
> -\subparagraph{TX Notifications}\label{sec:Device Types / Network Device /
> Device Operation / Control Virtqueue / Notifications Coalescing / TX
> Notifications}
> +\subparagraph{TX Example}\label{sec:Device Types / Network Device / Device
> Operation / Control Virtqueue / Notifications Coalescing / TX Example}
>
> If, for example:
> \begin{itemize}
> -\item \field{tx_usecs} = 10.
> -\item \field{tx_max_packets} = 15.
> +\item \field{usecs} = 10.
> +\item \field{max_packets} = 15.
> \end{itemize}
>
> -The device will operate as follows:
> +A device with a single transmit virtqueue will operate as follows:
>
> \begin{itemize}
> \item The device will count sent packets until it accumulates 15, or until
> 10 usecs elapsed since the first one was sent.
> \item If the notifications are not suppressed by the driver, the device will
> send an used buffer notification, otherwise, the device will not send an used
> buffer notification as long as the notifications are suppressed.
> \end{itemize}
>
> +\subparagraph{Notifications When Coalescing Parameters
> Change}\label{sec:Device Types / Network Device / Device Operation / Control
> Virtqueue / Notifications Coalescing / Notifications When Coalescing
> Parameters Change}
> +
> +When a device changes the coalescing parameters, it needs to check if the
> new notification conditions are met and send a used buffer notification if so.
> +
> +For example, \field{max_packets} = 15 for a device with a single transmit
> virtqueue.
> +If the device sends 10 packets, then it receives a
> VIRTIO_NET_CTRL_NOTF_COAL_TX_SET command with \field{max_packets} = 8, the
> device needs to immediately send a used buffer notification, if the
> notifications are not suppressed by the driver.
> +
> \drivernormative{\subparagraph}{Notifications Coalescing}{Device Types /
> Network Device / Device Operation / Control Virtqueue / Notifications
> Coalescing}
>
> If the VIRTIO_NET_F_NOTF_COAL feature has not been negotiated, the driver
> MUST NOT issue VIRTIO_NET_CTRL_NOTF_COAL commands.
> @@ -1583,7 +1598,7 @@ \subsubsection{Control Virtqueue}\label{sec:Device
> Types / Network Device / Devi
>
> A device SHOULD respond to the VIRTIO_NET_CTRL_NOTF_COAL commands with
> VIRTIO_NET_ERR if it was not able to change the parameters.
>
> -A device SHOULD NOT send used buffer notifications to the driver, if the
> notifications are suppressed as explained in \ref{sec:Basic Facilities of a
> Virtio Device / Virtqueues / Used Buffer Notification Suppression}, even if
> the coalescing counters expired.
> +A device SHOULD NOT send used buffer notifications to the driver if the
> notifications are suppressed, even if the notification conditions are met.
>
> Upon reset, a device MUST initialize all coalescing parameters to 0.
--
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